This is ripping off the thread asking for your top 100 sitcoms over in the British sitcom forum, only this time it's for American sitcoms only.
Granted, it's probably difficult to get anywhere near a top 100, but I'm going to try and think up my top twenty to start with.
Anyone willing to kick us off?
A number in no particular order:
Becker
The King of Queens
Frasier
30 Rock
The Big Bang Theory
Samantha Who?
Everybody Loves Raymond
Cybill
Seinfeld
Family Guy
Curb Your Enthusiasm
The Simpsons
Friends
And not really a sitcom but still funny and worthy of an honourable mention, Moonlighting.
1. Friends
2. Frasier
3. Arrested Development
4. Seinfeld
5. Curb Your Enthusiasm
6. Fresh Prince
7. Cheers
8. Scrubs
9. Kenan & Kel
10. ?
Super quick top 10, I'm sure I'm forgetting loads.
Off the top of my head, sure I've missed some out:
1. Friends
2. Two and A Half Men
3. Fresh Prince of Bel Air
4. Frasier
5. Scrubs
6. My Wife & Kids
7. Spin City
8. South Park
9. Kenan & Kel
10. Ellen
11. Futurama (these last three would be higher but I can't be bothered to reorder them just yet)
12. Two Guys and a Girl
13. Saved By The Bell
...can't think of any others I really like at the moment.
I don't really like that many US sitcoms, so I can only say:
Cheers
The Simpsons
Happy Days
Fresh Prince of Bel Air
I used to like Taxi when I was younger, but when I watch it now it seems a bit soppy. Maybe Mork and Mindy is still good, but I haven't seen that in years.
Okay, I've got a top eleven, I just couldn't force it down to ten.
1)Seinfeld
2)Mash
3)Cheers
4)Friends
5)Frasier
6)Curb your Enthusiasm
7)The King of Queens
8)Arrested Development
9)Everybody Loves Raymond
10)The Office US
11)The Cosby Show
I suppose I could have knocked Cosby off the bottom of the list to make it neater, but he has to stay there for sentimental reasons. The Cosby Show was such a huge part of my television viewing growing up after all.
I'll include The Simpsons, Family Guy and South Park if they count then.
Quote: Leevil @ June 11 2008, 3:10 PM BSTI'll include The Simpsons, Family Guy and South Park if they count then.
I know a lot of people do count them, but I don't personally
Anyway, if I did, Futurama and King of the Hill would get my votes over the above three 
Of course King of the Hill >_< How could I forget and Futurama too
King of the Hill is baaaaad!
I'll add Spin City though. Great show.
Ooh, and Jake in Progress.
Quote: Aaron @ June 11 2008, 3:23 PM BSTKing of the Hill is baaaaad!
That boy ain't right.
Here's my Top 5
1. Bilko
2. Bilko
3. Bilko
4. Bilko
5. Bilko
The Phil Silvers Show* x5
Quote: Aaron @ June 11 2008, 3:38 PM BSTThe Phil Silvers Show* x5
Yes, yes, yes you pedantic arse! 
I hereby go to the back of the class.
I am not up with the current US sitcoms, so I do not yet know about the delights of Curb your Enthusiasm or Arrested Development, so heres my list of 'older' favourites:
In no particular order -
Police Squad
Sledgehammer
Soap
The Phil Silvers Show
Friends
Frasier
Married With Children
WKRP in Cincinnati
Fresh Prince of Bel Air
Who's The Boss?
Different Strokes
The Gary Shandling Show
Taxi
Hang Time
Golden Girls
Def.